The studies will be carried out in patients with chronic renal disease in order to investigate the metabolism of fluoride in patients in whom the function of the major organ of fluoride excretion, the kidney, is impaired. The dietary fluoride intake and the renal and intestinal excretions of fluoride will be determined for several weeks under strictly controlled dietary conditions in the Metabolic Research Ward in order to assess the retention of fluoride under dietary conditions which are appropriate for these patients. The fluoride intake will be due to the fluoride content of the diet and the intake of fluoridated drinking water. These studies will be carried out during a normal calcium intake and during the addition of calcium to the constant diet. Attempts will be made to study the changes in fluoride metabolism during different phases of renal impairment in the same subject in order to correlate the changes in fluoride metabolism with the extent of renal impairment. The effect of medications which are commonly used for the treatment of these patients will be investigated on the metabolism of fluoride of these patients. Examples of these drugs are diuretics and antihypertensive compounds, used alone or in conjunction with the antacid aluminum hydroxide which is commonly used to decrease the elevated serum phosphorus level in patients with chronic renal disease.